CN114640391B - Mixed constellation shaping method of DPSK system facing FSO channel variation - Google Patents

Abstract

The invention discloses a mixed constellation shaping method of a high-order optical DPSK system facing FSO channel change, and belongs to the technical field of free space optical communication. Based on a high-order optical DPSK traditional constellation diagram, the demodulation complexity of certain bit information is reduced through geometric constellation shaping, GCS auxiliary bit marking geometric shaping information is adopted, the GCS auxiliary information is transmitted through amplitude modulation of an optical signal, meanwhile, the probability of distribution of low-amplitude constellation points is reduced according to different channel conditions by adopting a probability constellation shaping technology, the probability of distribution of high-amplitude constellation points is improved, or the probability of distribution of high-amplitude constellation points is reduced, and the probability of distribution of low-amplitude constellation points is improved. And adopting the multilevel PAM to realize the high-order DPSK signal transmission of mixed constellation shaping. In the FSO channel, the PCS+GCS mixed constellation shaping is adopted, so that the attenuation resistance or turbulence resistance of the optical high-order DPSK system under different channel conditions can be improved, and the system reliability is improved.

Description

Mixed constellation shaping method of DPSK system facing FSO channel variation

Technical Field

The invention belongs to the technical field of free space optical communication (Free Space Optics, FSO), and relates to a mixed constellation shaping method design of high-order differential phase shift keying (Differential Phase Shift Keying, DPSK) based on optical communication.

Background

With the rapid development of network technology and communication technology, the construction of global stereoscopic, high-speed, safe and reliable informationized networks becomes a new hot spot of scientific research nowadays, and the creation of high-speed wireless communication networks covering the world is an important component for realizing the aim. The rapid development of wireless communication technology has increased demands for high-capacity, high-rate and high-stability wireless communication, and conventional Radio Frequency (RF) communication spectrum resources are gradually limited, so that the increasing network demands are difficult to meet. Therefore, free space optical communication technology has been developed, and FSO communication technology adopts laser as a transmission carrier, and uses free space as a transmission medium, so that high-capacity high-speed information transmission can be realized, spectrum permission is not needed, and the defect of RF communication can be overcome. The FSO communication technology is a product of combining the optical communication technology and the wireless communication technology, and can make up for the existing disadvantages of the optical communication and the wireless communication while having the advantages of the optical communication and the wireless communication. FSO communication is used as one of the key technologies of the national space-world integrated development strategy, and near-infrared bands are generally used near the ground and in space, so that the FSO communication has rich frequency spectrum resources. Compared with RF communication and optical fiber communication, the FSO communication technology adopts laser as a transmission carrier, and the atmosphere is a transmission medium, so that the FSO communication technology has the advantages of stronger anti-electromagnetic interference capability, higher transmission rate, more flexible erection, better confidentiality, stronger directivity, and great application prospect in the fields of military field, emergency communication field, deep space communication field, space-earth integrated construction and the like.

The development of FSO communication technology is also limited by a number of factors. Because the laser beam is narrower, the anti-interference capability and confidentiality are improved, and meanwhile, the alignment requirement on the optical transceiver antenna of the communication system is high. Meanwhile, absorption and scattering effects of atmospheric gas molecules and aerosol particles in an atmospheric channel cause light intensity attenuation, and non-uniformity of air refractive index causes light intensity fluctuation and phase fluctuation. Thus, the communication quality of FSO is significantly affected by the atmospheric channel conditions. The DPSK modulation technology is a modulation technology commonly used in FSO because the DPSK modulation technology can avoid the phase ambiguity phenomenon and has 3dB receiving sensitivity improvement compared with OOK modulation, and particularly the DPSK modulation technology with higher order has higher frequency spectrum efficiency, so the DPSK modulation technology has higher research value. However, as the modulation order of the conventional high-order DPSK system increases, the system structure of the conventional high-order DPSK system is more and more complex, and particularly at the receiving end of the system, more differential demodulation devices and more complex logic decision circuits are often required, which brings a certain problem to the feasibility and reliability of the system. The prior research mainly obtains the same information rate by an amplitude-phase joint modulation method, thereby achieving the purpose of reducing the complexity of the system structure. Because amplitude modulation is often severely affected by an atmospheric channel, both the atmospheric turbulence and attenuation effects degrade the signal communication quality based on the amplitude modulation, which is unfavorable for transmission in an atmospheric channel with medium-high turbulence intensity or greater attenuation. The constellation shaping technology is a hot research technology in the field of optical fiber communication at present because of the advantages of resisting optical fiber nonlinearity and improving system capacity, and can be divided into geometric constellation shaping technology (Geometric Constellation Shaping, GCS) and probability constellation shaping (Probabilistic Constellation Shaping, PCS) technology. Geometric constellation shaping is to change the spatial position of constellation points so that the spatial distribution of the constellation points is Gaussian distribution to approach Shannon limit and improve system capacity; probability shaping technology is commonly used for Quadrature Amplitude Modulation (QAM) signals, and probability distribution of constellation points is changed to enable the probability of the constellation points to be Gaussian distribution by taking the origin of constellation diagram coordinates as the center, so that average symbol energy is reduced, and system reliability is improved. But PCS technology cannot be used for conventional high-order optical DPSK signals because of the constant amplitude of the optical DPSK signal. In addition, unlike the optical fiber, the air turbulence peculiar to the FSO channel also causes fluctuation of light intensity and phase fluctuation, and further causes increase of Bit Error Rate (BER) of the system. Therefore, how to combine the constellation shaping technology and how the constellation shaping technology has performance improvement for the FSO communication system in the FSO communication system is worth researching.

Through retrieval, application publication number CN112737686B, a high-performance spatial light transmission system based on geometric probability shaping technology, which is characterized in that the spatial light transmission system comprises a digital signal processing module and an optical transmission module; the digital signal processing module comprises a distribution adapter, a constellation mapping unit, an up-sampling unit, a shaping filter and an adder unit; the distribution adapter is used for converting binary data into a distribution form after probability shaping calculation; the constellation mapping unit is used for performing constellation mapping according to the three-dimensional constellation diagram, and performing geometric shaping on constellation points in the three-dimensional space. The invention can apply probability shaping and three-dimensional geometric shaping to the electric signal at the transmitting end part of the space optical transmission system, and can cope with the influence caused by atmospheric turbulence by improving the anti-interference capability of the signal at the transmitting end, thereby effectively improving the anti-noise capability of the system, weakening the influence caused by the atmospheric turbulence effect and improving the frequency spectrum utilization rate and the transmission rate of the system to a certain extent.

The geometric constellation shaping scheme and the probability constellation shaping scheme designed in the invention are different from the shaping scheme in the patent, and the geometric shaping is performed in two dimensions based on the distribution characteristics of the constellation diagram and the demodulation principle of the high-order DPSK signal, so that the high-order DPSK constellation diagram is shaped in phase and amplitude. Meanwhile, according to different channel conditions, the probability constellation shaping scheme designed by the invention does not concentrate the distribution probability to low-amplitude constellation points all the time, and the PCS scheme designed by the invention can also improve the anti-atmospheric attenuation performance of the system when more distribution probability of constellation points is concentrated to high amplitude under the atmospheric channel conditions of low turbulence and high attenuation. Therefore, the invention has the advantages that the designed mixed constellation shaping method can improve the light receiving sensitivity when the high-order DPSK system faces to the FSO channel change, so that the system achieves good communication quality, and the atmospheric turbulence resistance or the atmospheric attenuation resistance of the system can be pertinently improved.

Disclosure of Invention

The present invention is directed to solving the above problems of the prior art. A mixed constellation shaping method of a DPSK system facing FSO channel variation is provided. The technical scheme of the invention is as follows:

a hybrid constellation shaping method of a FSO channel variation oriented DPSK system, comprising the steps of:

the steps of the designed geometric constellation shaping GCS scheme are as follows: the geometric constellation shaping GCS scheme is realized by adopting geometric shaping and auxiliary bit marking; the geometric constellation shaping scheme is divided into two steps: 1) Shaping the phase of constellation points: by changing constellation mapping relation; 2) Shaping the amplitude of constellation points: by means of auxiliary bit amplitude modulation. The GCS constellation diagram is obtained through the two steps;

a step of designing a probability constellation shaping PCS scheme;

transmitting by adopting a GCS+PCS mixed shaping signal mode;

and demodulating the GCS+PCS mixed shaping signal.

Further, the steps of the designed geometric constellation shaping GCS scheme specifically include: 1) For a traditional high-order optical DPSK constellation diagram, the bit number corresponding to each symbol is determined by the modulation order, and the bit refers to a first bit in each symbol, so that a bit with higher distribution complexity of the constellation diagram is searched; 2) The constellation points corresponding to the bits with higher distribution complexity of the constellation diagram are subjected to phase shaping of the constellation points, and the distribution complexity of the bits is reduced by changing the mapping relation of the constellation points; 3) The symbol of the 2 nd stepping planet seat point phase shaping is marked with GCS auxiliary bits to mark whether the constellation points are subjected to mapping relation transformation or not, the GCS auxiliary information enables the constellation points to be subjected to amplitude shaping through amplitude modulation, and 4) the GCS constellation diagram is obtained after the steps 1, 2 and 3.

Further, the steps of the designed probability constellation shaping PCS scheme specifically include: 1) Under the condition of a low-turbulence high-attenuation atmospheric channel, shaping constellation symbols with low amplitude on a high-order DPSK constellation diagram after GCS to constellation diagram high-amplitude constellation points according to a certain probability, and improving the distribution probability of the high-amplitude constellation points; 2) Under the atmospheric channel condition of high turbulence and low attenuation, high-amplitude constellation symbols on a high-order DPSK constellation diagram after GCS are shaped to constellation diagram low-amplitude constellation points according to a certain probability, and the distribution probability of the low-amplitude constellation points is improved.

Further, the transmission mode of the gcs+pcs mixed shaping signal specifically includes:

PCS auxiliary marking is carried out on each constellation symbol after GCS whether probability shaping is carried out or not; the optical signal needs to transmit GCS and PCS auxiliary bit information, and because the PCS design scheme is based on the premise that the number of constellation diagram amplitude values after GCS is unchanged, bipolar multi-system Pulse Amplitude Modulation (PAM) is adopted, the GCS auxiliary bit information is modulated onto the amplitude of the optical signal through the PAM, and the PCS auxiliary bit information is modulated onto the phase of the optical signal through phase shift generating pi, so that the transmission of GCS+PCS mixed shaping signals in an FSO channel is realized.

Further, the specific steps in the PCS scheme of the bipolar multilevel pulse amplitude modulation PAM include: 1) The GCS auxiliary bit information is represented by the positive amplitude of a bipolar PAM signal; the PCS auxiliary bit information is represented by the negative amplitude of the PAM signal, and the negative amplitude is equal to the maximum amplitude of the positive electrode; 2) Taking the PAM signal as the driving voltage of the MZM of the Mach-Zehnder modulator, and taking the traditional high-order optical DPSK signal as the input optical signal of the MZM; 3) And placing the bias point of the MZM at the lowest point of a transmission curve, performing amplitude modulation on the positive electrode of the PAM signal, performing pi phase shift on the negative electrode, and enabling the amplitude to be equal to the maximum amplitude of the positive electrode, thereby realizing the modulation of the PAM.

Further, the specific steps of the bipolar multilevel pulse amplitude modulation PAM in the PCS scheme of step 2 include: 1) The GCS auxiliary bit information is represented by the positive amplitude of a bipolar PAM signal; the PCS auxiliary bit information is represented by the negative amplitude of the PAM signal, and the negative amplitude is equal to the minimum amplitude of the positive electrode; 2) Taking the PAM signal as the driving voltage of the MZM of the Mach-Zehnder modulator, and taking the traditional high-order optical DPSK signal as the input optical signal of the MZM; 3) And placing the bias point of the MZM at the lowest point of a transmission curve, performing amplitude modulation on the positive electrode of the PAM signal, performing pi phase shift on the negative electrode, and enabling the amplitude to be equal to the minimum amplitude of the positive electrode, thereby realizing the modulation of the PAM.

Further, the system demodulation end structure of the mixed constellation shaping signal is as follows: differential phase demodulation is carried out on the high-order optical DPSK signal after constellation reconstruction by adopting a Mach-Zehnder interferometer MZI, and a phase demodulation branch consists of one MZI, two photodiodes, one subtracter, one low-pass filter and one decision device; the MZI is used for differential demodulation of signals, the photodiode is used for photoelectric detection of the demodulated signals, the subtracter is used for demodulating differential information, the low-pass filter is used for filtering low-frequency noise, and the decision device is used for restoring signals.

The PAM demodulation branch adopts coherent demodulation and consists of a laser source, a DSP module, a 3dB coupler, two photodiodes, a subtracter, a low-pass filter and a decision device; the laser source and the 3dB coupler are used for coherent demodulation, and the DSP module is used for compensating phase noise;

the demodulation end system after PCS+GCS consists of a plurality of phase demodulation branches, a PAM coherent demodulation branch and related logic decision circuits.

Further, the system demodulation step of the mixed constellation shaping signal is as follows: 1) The system after GCS realizes the demodulation of partial bit through the phase demodulation branch and simple logic operation according to the distribution condition of each bit of the GCS constellation diagram; 2) Demodulation of the residual bit is realized by demodulating GCS auxiliary information and PCS auxiliary information through a PAM demodulation branch and carrying out simple logic operation on the auxiliary information and the demodulated bit information; 3) And converting the demodulated parallel bits into serial output, so as to restore the original signal and complete the information transmission of the mixed constellation shaping system.

The invention has the advantages and beneficial effects as follows:

aiming at the defects of the existing research of a high-order optical DPSK system in FSO communication, the invention provides a mixed constellation shaping method of the high-order optical DPSK system based on FSO communication, which carries out geometric constellation shaping on bit with complex demodulation according to constellation diagram and demodulation principle, adjusts the mapping relation of the bit corresponding to constellation points, reduces the bit distribution complexity and simplifies the demodulation principle of the bit. Meanwhile, the symbols for GCS are marked with auxiliary bits, and the GCS auxiliary bits are transmitted by amplitude modulation. At this time, the constellation diagram of the optical signal transmitted in the FSO channel presents a plurality of amplitude values, so that the PCS technology can be adopted to carry out probability shaping on constellation points on different amplitude values, different PCS schemes are designed aiming at different atmospheric channel conditions, and the constellation symbols of which the transmitting end needs to carry out probability shaping are subjected to PCS auxiliary bit marking and combined with the original GCS auxiliary bit for transmission. Because the PCS scheme is carried out on the basis of the original GCS scheme, the number of signal amplitude values after GCS cannot be changed in the transmission process, the invention adopts the multi-system pulse amplitude modulation to transmit the auxiliary bits, thereby realizing the simultaneous transmission of two auxiliary bits under the condition of unchanged amplitude order. The PCS technology is adopted to reshape the high-order DPSK constellation again, so that the influence caused by channels under different atmospheric conditions can be properly dealt with, and the system performance is further improved.

The invention provides a mixed constellation shaping method of a DPSK system facing FSO channel change, which mainly comprises the following steps: by designing the geometric constellation shaping scheme and the probability constellation shaping scheme according to claims 2 and 3, different hybrid constellation shaping methods according to claim 3 are performed on optical signal constellations transmitted by the high-order DPSK system in different FSO channels. The auxiliary labeling methods in claims 2 and 4 are designed such that the signal can be demodulated for a mixed shaping constellation. Meanwhile, the bipolar PAM technology in claims 5 and 6 is adopted, and the geometric constellation shaping scheme and the probability shaping scheme are skillfully combined, so that the transmission of the mixed constellation shaping signal is realized. According to the channel characteristics of FSO, different mixed constellation shaping schemes can be designed to adaptively improve the attenuation resistance or turbulence resistance of the high-order optical DPSK system according to the characteristics of the channel. The mixed constellation shaping method provided by the invention can improve the receiving sensitivity of the high-order optical DPSK system, so that the system has good communication quality under different FSO channel conditions.

Drawings

Fig. 1 is a hybrid constellation shaped optical 16DPSK modulation system architecture;

FIG. 2 is a schematic diagram of MZM modulation;

FIG. 3 is a diagram of a conventional optical 16DPSK demodulation system;

FIG. 4 is a conventional optical 16DPSK signal constellation;

FIG. 5 is an optical 16DPSK signal constellation after GCS;

fig. 6 is a diagram of an optical 16DPSK demodulation system architecture after mixed constellation shaping;

FIG. 7 is a BER comparison of different bits as a function of received optical power before and after mixed constellation shaping for an optical 16DPSK system;

FIG. 8 is a system BER comparison of the light 16DPSK system with the change of the received light power before and after the mixed constellation shaping;

FIG. 9 is a comparison of system BER as a function of laser transmit power before and after mixed constellation shaping for an optical 16DPSK system;

FIG. 10 shows BER of each bit under different turbulence intensities of a conventional optical 16DPSK system and an optimized system after mixed constellation shaping when the received optical power is the same;

FIG. 11 shows the BER of the optical 16DPSK conventional system and the optimized system after mixed constellation shaping under different turbulence intensities when the received optical power is the same;

FIG. 12 shows BER of each bit under different turbulence intensities of a traditional optical 16DPSK system and an optimized system after mixed constellation shaping when the laser emission power and the laser emission power are the same;

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and specifically described below with reference to the drawings in the embodiments of the present invention. The described embodiments are only a few embodiments of the present invention.

The technical scheme for solving the technical problems is as follows:

the high-order optical DPSK system mainly adopts Mach-Zehnder Modulator, MZM to carry out photoelectric phase modulation, and the corresponding demodulation device is Mach-Zehnder Interferometer, MZI. The MZI number N and modulation order M (m=2 ⁿ N.gtoreq.2) has the following relationship:therefore, as the modulation order increases, the system structure and logic decision rule become more complex, so that the reliability of the system is reduced, mainly because of the increase of constellation points of the DPSK signal, the more complex the distribution of each bit becomes, the smaller the minimum Euclidean distance of the corresponding constellation point is reduced, and the error condition is easily caused by the influence of a channel.

Based on the above objective, the present invention proposes a GCS method according to the demodulation principle and constellation distribution of the high-order DPSK system, and distributes constellation points of the high-order DPSK on a plurality of magnitudes, so as to reduce average symbol energy. The constellation mapping relation transformation is carried out on constellation points with more complex bit distribution, so that the distribution condition of the bits is adjusted, meanwhile, GCS auxiliary marking is carried out on constellation symbols subjected to mapping transformation, and the amplitude modulation is adopted to transmit GCS auxiliary bit information, so that the constellation point transformation information is loaded to the signal amplitude. Geometric constellation shaping of the high-order DPSK signal is realized.

The constellation diagram after GCS presents various amplitudes, the distance between constellation points with lower amplitudes is reduced while the average symbol energy is reduced, so that the error rate of low-amplitude symbols is increased when the atmospheric attenuation is higher, and the capability of high-amplitude constellation symbols for resisting attenuation is stronger than that of the traditional high-order DPSK constellation diagram due to the reduced distance between constellation points; meanwhile, the amplitude fluctuation of the low-amplitude constellation symbol is smaller due to the fact that the amplitude of the low-amplitude constellation symbol is lower, so that the low-amplitude constellation symbol is higher in resistance to the atmospheric turbulence, and the high-amplitude constellation symbol is influenced by the atmospheric turbulence, and is larger in amplitude fluctuation, so that the bit error rate of the high-amplitude constellation symbol is higher when the atmospheric turbulence is higher. Thus, for this case, the PCS technique may be employed to probability shape constellation symbols of different magnitudes when dealing with different channel conditions. When the atmospheric channel is low-attenuation high-turbulence, the PCS scheme is designed so that the occurrence probability of constellation points is increased along with the reduction of the amplitude, and more constellation symbols are concentrated towards the low amplitude so as to resist the influence caused by the high turbulence under the condition of lower attenuation; when the atmosphere channel is high-attenuation low-turbulence, the PCS scheme is designed so that the occurrence probability of constellation points is increased along with the increase of the amplitude, and more constellation symbols are concentrated towards the high amplitude so as to resist the influence caused by high attenuation under the condition of low turbulence.

Based on the above purpose, the invention carries out PCS auxiliary marking on the symbol needing to carry out probability shaping at the transmitting end after geometric constellation shaping, and combines with GCS auxiliary bit to form a multi-system information sequence. If common amplitude modulation is adopted, the amplitude of the constellation diagram is increased, which contradicts with the PCS scheme, so that the information transmission of the PCS auxiliary bit is realized while the amplitude order of the constellation diagram after GCS is ensured to be unchanged. The invention adopts the multi-system pulse amplitude modulation technology, realizes the transmission of GCS auxiliary information on the loading amplitude, and simultaneously, PCS auxiliary information is loaded on the phase to be transmitted along with the same optical signal. After the symbol subjected to PCS realizes pi phase shift through MPAM modulation, the symbol is represented on a constellation diagram to fall on a constellation symbol position of another amplitude value with pi angle phase difference, so that the change of probability distribution is realized.

The invention provides a GCS and PCS mixed constellation shaping method aiming at a high-order DPSK system in FSO communication, and the combination of the two constellation shaping methods is realized through an MPAM modulation technology. The mixed constellation shaping method in the invention is different from constellation shaping in optical fiber communication from the angles of Euclidean distance of constellation points and FSO channel characteristics, and in an FSO communication system, the constellation shaping is mainly used for resisting the influence of different channel conditions, so that the starting point of constellation shaping design is not used for resisting optical fiber nonlinearity any more, and the method has great research significance for improving the performance of the FSO communication system.

The invention has been verified in a 16DPSK system in FSO communication, and the specific implementation process of the mixed constellation shaping method in the optical 16DPSK system is as follows:

for the transmitting end, the structure of the 16DPSK system subjected to mixed constellation shaping is similar to the conventional structure, as shown in fig. 1. The main difference is that the transmitting end needs to use one MZ modulator to realize PAM modulation, and the following is the generation process of the transmitting signal:

1. the initial sequence is subjected to serial-parallel conversion and then differential pre-coding, and the initial sequence is coded into a relative code representing the difference between two adjacent symbols;

2. based on the demodulation principle of the optical 16DPSK signal, the transmitting end carries out geometric constellation shaping on the 16DPSK signal constellation diagram, and the GCS scheme mainly changes the mapping relation of constellation points where bits with complex demodulation process are located, in the example, the distribution of c bits on the constellation diagram is changed, so that 0 and 1 of the c bits are distributed in half;

3. performing GCS auxiliary marking on the symbol subjected to constellation mapping transformation, wherein the number of the GCS auxiliary bits is two, so that two magnitudes exist in the 16DPSK constellation point after shaping;

4. at the same time, the 16DPSK signal is probability shaped in combination with PCS technology. The symbol subjected to probability shaping is subjected to shaping bit marking at a transmitting end and marked as PCS auxiliary bits;

the PCS and GCS auxiliary mark bits are combined to form a ternary information sequence. 11, 10, 00, respectively. Wherein 11 represents an original high-amplitude symbol; 10 denotes a probability shaped symbol, possibly from a high amplitude to a low amplitude, or from a low amplitude to a high amplitude; 00 represents the original low-amplitude symbol; because the auxiliary mark bit determines that the 16DPSK signal can only have two amplitude values, a PAM modulation format can be adopted to realize ternary information transmission, so that the signal can transmit auxiliary bit information on the amplitude values and simultaneously transmit shaping bit information on the phase.

6. The ternary PAM signal is loaded onto the same optical wave along with the 16DPSK signal, with the GCS auxiliary bit information modulated onto the amplitude of the optical signal; after the symbol subjected to PCS realizes pi phase shift through PAM modulation, the symbol is represented on a constellation diagram to fall on the symbol position of another constellation, so that the probability distribution is changed. The 16DPSK is output in combination with a ternary PAM modulated signal, and the modulation principle of the ternary PAM signal on the MZM is shown as shown in figure 2, so that the shaping of the low-amplitude symbol into a high-amplitude symbol with pi phase difference is realized.

The optical signal reaches the receiving end after passing through the FSO channel, amplified by the optical amplifier, filtered by the low-pass filter to remove part of background optical noise, the optical splitter distributes the received optical signal equal power to the demodulation end, the demodulation end demodulates the 16DPSK after constellation shaping, and the signal constellation diagram is different from the traditional demodulation of the 16DPSK because the signal constellation diagram passes through the GCS and the PCS, and the demodulation structure of the traditional 16DPSK system is shown in fig. 3. Six branches with similar structures are needed at the demodulation end of the traditional 16DPSK system, each branch is subjected to interference demodulation by one MZI to demodulate phase information, a balance detector is used for photoelectric detection, an optical signal is converted into an electric signal to be output, a differential electric signal is obtained through a subtracter, and then a bit sequence is output through a low-pass filter and a decision device. Finally, the initial bits are restored through a series of logic operations. Wherein the MZI phase shift is different for each branch, depending mainly on the constellation of 16DPSK, as shown in fig. 4. The phase shift causes the constellation to be rotated counter-clockwise along the horizontal axis by a corresponding angle, which is equivalent to rotating the horizontal axis clockwise by a corresponding angle. The phase shift of the first branch enables the constellation diagram to directly demodulate a bit information after rotation, the second branch can demodulate b bit information, and the c bit information is obtained by corresponding logic operation of the a bit and the b bit. The demodulation of d-bit information is relatively complex and requires the third to sixth branches to be processed together by corresponding logic operations.

Thus, the constellation diagram through the GCS is shown in fig. 5, where the constellation points are distributed over two magnitudes, and, according to the PCS scheme, the probability distribution of the constellation points in this example is: 1) The occurrence probability of the low-amplitude constellation symbols is 25%, and the occurrence probability of the high-amplitude constellation symbols is 75%; 2) The probability of occurrence of the low-amplitude constellation symbols is 75%, and the probability of occurrence of the high-amplitude constellation symbols is 25%. Therefore, as shown in fig. 6, the demodulation structure of the constellation-shaped 16DPSK signal is as follows:

1. the demodulation end consists of two MZI branches with the same structure and one coherent demodulation branch. For demodulation of constellation points, the first MZI branch and the second MZI branch which assume that the shaped constellation point symbols are a ' b ' c'd ' respectively demodulate the a ' bit information and the c ' bit information, and the d ' bit information is obtained by simple logic operation of the obtained a ' bit information and the c ' bit information:. And at this point the b 'bit can be solved by exclusive or of c' with the GCS auxiliary bit.

And 2, performing coherent demodulation on PAM to obtain initial ternary auxiliary information, and obtaining high-low amplitude information and probability constellation shaping information according to the auxiliary information. Wherein the high and low amplitude information is co-processed with c 'or can be solved for b' information.

3.b 'and d' are equal to the initial bit sequences b, d before shaping, respectively; the a ', c' bit information is inverted on the symbol bits subjected to probability constellation shaping, and the a and c initial bit information can be restored.

The conventional optical 16DPSK system and the optical 16DPSK system after constellation shaping using the proposed invention were simulated and tested for system performance by optistystem software according to the parameter settings of table 1.

Fig. 7 compares BER of various bits of information for a conventional optical 16DPSK system and a pcs+gcs optimized system for an atmospheric channel under low turbulence and high attenuation. When the atmospheric turbulence intensity is 1X 10 ^-16 m ^-23 In the process, the adopted GCS+PCS mixed constellation shaped optical 16DPSK system can be found, the received optical power of the system is reduced along with the increase of atmospheric attenuation, the BER of each bit of information of the system is lower than that of the traditional system, and the improvement of the reliability of each bit is further obtained compared with an optimized system adopting only geometric constellation shaping. Since the PCS scheme at this time concentrates constellation symbols appearing at low amplitudes toward high amplitudes in order to resist the atmospheric channel condition of low turbulence and high attenuation, the probability of distribution of low amplitude constellation points at this time is 0.25, and the probability of distribution of high amplitude constellation points is 0.75. The amplitude fluctuation of the high-amplitude symbols brought by the low-turbulence atmospheric environment is small, and more high-amplitude symbols are favorable for resisting the optical power loss brought by high attenuation, so that the BER performance of each bit of the optical 16DPSK system after the mixed constellation shaping is improved.

The conventional system has the same demodulation structure of a bit and b bit and the lowest complexity, so the BER of the a bit and the b bit is minimum in all bits, and the demodulation structure of d bit needs four demodulation branches, and the decision domain is smaller, so the BER of d bit information is maximum.

For the optical 16DPSK system after mixed constellation shaping optimization, the geometric distribution of constellation points is changed, so that the demodulation structure originally used for demodulating b bits is used for demodulating c bit information at the moment, the demodulation structures of a and c bits of the optimization system are the same and have the lowest complexity, the BER curves of the a and c bits are similar and the minimum, b bits are obtained by the same or the same GCS auxiliary information and the c bit information, the reliability of GCS signals is further improved by adopting coherent demodulation, the BER of the GCS auxiliary information is smaller, and the BER of the b bit information of the system is close to the BER of the c bit information. The d bit demodulation structure at this time is the same as the c bit demodulation structure of the traditional system, and meets the following requirements. Therefore, the BER curve of the d-bit information of the optimized system is similar to the BER curve trend of the c-bit information of the traditional system, and is the bit sequence with the highest BER in the optimized system.

The BER of the whole system is the integration of the BER of each bit of information, and fig. 8 compares the system BER of the conventional optical 16DPSK system with the system BER of the optimization system using the mixed constellation shaping of pcs+gcs. As can be seen from fig. 8, the optimized system after shaping the mixed constellation has higher communication quality in the low-turbulence high-attenuation air channel, and the BER of the system is significantly lower than that of the conventional system. The method mainly comprises the steps that an optimization system adopts GCS to divide constellation symbols into high and low amplitude values, under the same received light power, the high amplitude value of a signal after the receiving end adopts GCS can be higher than the signal amplitude value of a traditional system, the low amplitude value of the signal after the GCS can be lower than the amplitude value of the traditional system, the system structure is greatly simplified, and the reliability of demodulation of each bit is further improved. PCS is added on the basis of GCS, so that the symbols with low amplitude are shaped into high-amplitude constellation symbols with phase difference pi according to the probability of 0.5, the distribution probability of the high-amplitude constellation symbols is 0.75, and the distribution probability of the low-amplitude constellation symbols is 0.25, and therefore the optical power of the high-amplitude symbols is increased, and the capability of resisting channel attenuation is enhanced. Therefore, the PCS+GCS mixed shaping can improve the reliability of the system under the FSO channel condition of low turbulence and high attenuation.

Fig. 9 compares the system BER as the laser transmit power varies for a conventional optical 16DPSK system and a hybrid constellation shaping system. It can be seen that the BER of both systems decreases as the laser transmit power increases. And under the same laser emission power, the BER of the optimized system after PCS+GCS mixed constellation shaping is obviously lower than that of the traditional system. Although for the optimization system of PCS+GCS mixed constellation shaping, as constellation symbols with two amplitudes exist, the constellation symbols with low amplitude have lost a part of optical power during modulation, so that the average symbol optical power of the optimization system is reduced, the structure of a demodulation end of the system brought by GCS is simplified, and the turbulence resistance of the system brought by increasing the probability of high-amplitude symbol distribution by PCS is enhanced, so that the overall reliability performance of the optimization system is improved, which means that the optimization system has higher energy efficiency and can obtain higher system BER performance with lower average symbol energy.

Moreover, as can be further analyzed from fig. 9, when the same BER requirement is met, the laser emission power required by the optimizing system for mixed constellation shaping is lower than that of the conventional system, which further illustrates that the optimizing system can use lower laser emission power to achieve the same communication quality, and the laser emission power is saved.

Likewise, the transmission of signals of both systems under high turbulence low attenuation atmospheric channel conditions was simulated in optistystem according to the parameter settings of table 2.

Fig. 10 compares the BER of bits at different turbulence intensities for a conventional system of optical 16DPSK and an optimized system after mixed constellation shaping at a received optical power of-10 dBm. As can be seen from fig. 10, when the turbulence intensity is the same, the demodulation structures of the a bit and the b bit in the conventional system are the same and the complexity is the lowest, so the BER curves of the a bit and the b bit are similar and the minimum, and the BER increases sequentially with the increase of the complexity of the c demodulation and the d demodulation. As turbulence increases, the BER of each bit of the conventional system increases. For the optimized system, the demodulation structures of the a bit and the c bit are the same and the complexity is the lowest, so the BER curves of the a bit and the c bit are similar and the minimum. Meanwhile, b bits of the optimization system are obtained by exclusive OR of c bits and the GCS auxiliary sequence, and the GCS auxiliary sequence has the advantage of improved reliability due to coherent demodulation, so that BER of b bit information is slightly higher than that of c bits. And the most complex d bits of bit information that becomes the highest BER of the optimized system are demodulated at this time. However, at the same turbulence intensity and received light power, each bit exhibits higher reliability because the optimization system drops more symbols at low amplitudes, improving the turbulence resistance of the low amplitude symbols.

Fig. 11 compares the system BER for the conventional system of optical 16DPSK and the optimized system after mixed constellation shaping at different turbulences. The results show that the BER of the hybrid constellation shaping optimization system is significantly lower than that of the conventional system when the received optical power is-10 dBm. This means that the optimized system has better reception sensitivity, and requires lower received optical power when the same BER performance is achieved. As the turbulence increases, the intensity fluctuation caused by the turbulence becomes intense, and the factor causing the error code is dominant by the turbulence. When the average received light power is the same, although the light intensity fluctuation of the high-amplitude symbol is more severe than that of the traditional system, the number of the high-amplitude symbol is reduced, more symbols are positioned at the low amplitude, so that the low-amplitude symbol is less influenced by turbulence, and the turbulence resistance caused by the high distribution probability of the low-amplitude symbol compensates the disadvantages that the power is lower and the high-amplitude symbol is easily influenced by turbulence, thereby representing lower BER as a whole.

Fig. 12 compares BER with turbulent flow for a conventional system and an optimized system at a laser emission power of 20 dBm. Unlike fig. 11, when the laser emission power is the same, the optimizing system shows two conditions of high and low in amplitude due to the mixed constellation shaping, the optical power of the high amplitude symbol is the same as that of the conventional system, and the amplitude modulation index is 0.5, so that the optical power of the low amplitude constellation symbol is half of that of the conventional system. The optimization system therefore loses a significant portion of its optical power in the modulation principle, making its average symbol power lower than in conventional systems. However, from the system BER situation shown in fig. 12, as the turbulence intensity increases, the system BER of the optimizing system is closer to that of the conventional system, and the optimizing system with lower average symbol power does not show obvious disadvantages, and it is illustrated from the side that the sensitivity of the hybrid constellation shaping optimizing system to optical power is more advantageous.

In summary, the hybrid constellation shaping method provided by the invention is suitable for a high-order optical DPSK system, and is used for improving the communication quality of optical high-order DPSK signals transmitted in a low-turbulence high-attenuation FSO channel. Based on the demodulation principle of the high-order optical DPSK system, the constellation points can be flexibly subjected to GCS and PCS transformation aiming at specific modulation orders, and the mixed constellation shaping scheme performed by the embodiment is not unique. The constellation points corresponding to bits with higher demodulation complexity are gathered by changing the mapping relation of constellation symbols, the positions of the constellation points are reasonably exchanged, and the exchange information is marked by GCS auxiliary bits, so that the aim of auxiliary demodulation is fulfilled. The structure of the system is simplified, and as the modulation order increases, the degree of simplification of the system structure increases. Meanwhile, probability shaping is carried out on constellation symbols after GCS, and different probability shaping schemes are designed according to different channel conditions, so that the anti-attenuation performance or the receiving sensitivity of the system is improved, and the optimization system can obtain good communication quality under different channel conditions. The invention also has a certain expansibility because the system reduces more structural complexity through mixed constellation shaping as the modulation order of the optical signal increases, and the obtained BER performance may further increase.

Table 1 is a table of system parameter settings for low turbulence and high attenuation channel conditions in accordance with an embodiment of the present invention;

table 2 shows a system parameter setting table under high turbulence and low attenuation channel conditions in accordance with an embodiment of the present invention

TABLE 1

TABLE 2

It should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article or apparatus that comprises the element.

The above examples should be understood as illustrative only and not limiting the scope of the invention. Various changes and modifications to the present invention may be made by one skilled in the art after reading the teachings herein, and such equivalent changes and modifications are intended to fall within the scope of the invention as defined in the appended claims.

Claims (2)

1. The mixed constellation shaping method of the DPSK system facing the FSO channel change is characterized by comprising the following steps of:

the steps of the designed geometric constellation shaping GCS scheme are as follows: the steps of the designed geometric constellation shaping GCS scheme specifically comprise: 1) For a traditional high-order optical DPSK constellation diagram, the bit number corresponding to each symbol is determined by the modulation order, and the bit refers to a first bit in each symbol, so that a bit with higher distribution complexity of the constellation diagram is searched; 2) The constellation points corresponding to the bits with higher distribution complexity of the constellation diagram are subjected to phase shaping of the constellation points, and the distribution complexity of the bits is reduced by changing the mapping relation of the constellation points; 3) Performing GCS auxiliary marking on the symbol subjected to phase shaping of the step-2 planet seat point to mark whether constellation points are subjected to mapping relation transformation or not, wherein the GCS auxiliary information enables the constellation points to be shaped in amplitude through amplitude modulation;

a step of designing a probability constellation shaping PCS scheme; the steps of the designed probability constellation shaping PCS scheme specifically comprise: 1) Under the condition of a low-turbulence high-attenuation atmospheric channel, shaping constellation symbols with low amplitude on a high-order DPSK constellation diagram after GCS to constellation diagram high-amplitude constellation points according to a certain probability, and improving the distribution probability of the high-amplitude constellation points; 2) Under the atmospheric channel condition of high turbulence and low attenuation, shaping constellation symbols with high amplitude on a high-order DPSK constellation diagram after GCS to constellation diagram low-amplitude constellation points according to a certain probability, and improving the distribution probability of the low-amplitude constellation points;

transmitting by adopting a GCS+PCS mixed shaping signal mode;

and demodulating the GCS+PCS mixed shaping signal;

the transmission mode of the GCS+PCS mixed shaping signal specifically comprises the following steps:

PCS auxiliary marking is carried out on each constellation symbol after GCS whether probability shaping is carried out or not; the bipolar multi-system Pulse Amplitude Modulation (PAM) is adopted, the GCS auxiliary information is modulated onto the amplitude of the optical signal through the PAM, and the PCS auxiliary information is modulated onto the phase of the optical signal through the phase shift generating pi, so that the transmission of the GCS+PCS mixed shaping signal in the FSO channel is realized;

the specific steps in the bipolar multi-system Pulse Amplitude Modulation (PAM) scheme comprise: 1) The GCS auxiliary information is represented by the positive amplitude of a bipolar PAM signal; the PCS auxiliary information is represented by the negative amplitude of the PAM signal, and the negative amplitude is equal to the maximum amplitude of the positive electrode; 2) Taking the PAM signal as the driving voltage of the MZM of the Mach-Zehnder modulator, and taking the traditional high-order optical DPSK signal as the input optical signal of the MZM; 3) The bias point of the MZM is arranged at the lowest point of a transmission curve, the positive electrode of the PAM signal carries out amplitude modulation, the negative electrode carries out pi phase shift, and the amplitude is equal to the maximum amplitude of the positive electrode, so that the PAM is modulated;

the system demodulation steps of the mixed constellation shaping signal are as follows: 1) The system after GCS realizes the demodulation of partial bit through the phase demodulation branch and simple logic operation according to the distribution condition of each bit of the GCS constellation diagram; 2) Demodulation of the residual bit is realized by demodulating GCS auxiliary information and PCS auxiliary information through a PAM demodulation branch and carrying out simple logic operation on the auxiliary information and the demodulated bit information; 3) And converting the demodulated parallel bits into serial output, so as to restore the original signal and complete the information transmission of the mixed constellation shaping system.

2. The method for shaping the mixed constellation of the DPSK system for FSO channel variation of claim 1, wherein the system demodulation end structure of the mixed constellation shaping signal is: differential phase demodulation is carried out on the high-order optical DPSK signal after constellation reconstruction by adopting a Mach-Zehnder interferometer MZI, and a phase demodulation branch consists of one MZI, two photodiodes, one subtracter, one low-pass filter and one decision device; the MZI is used for differential demodulation of signals, the photodiode is used for photoelectric detection of the demodulated signals, the subtracter is used for demodulating differential information, the low-pass filter is used for filtering low-frequency noise, and the decision device is used for restoring signals;

the PAM demodulation branch adopts coherent demodulation and consists of a laser source, a DSP module, a 3dB coupler, two photodiodes, a subtracter, a low-pass filter and a decision device; the laser source and the 3dB coupler are used for coherent demodulation, and the DSP module is used for compensating phase noise;

the demodulation end system after PCS+GCS consists of a plurality of phase demodulation branches, a PAM coherent demodulation branch and related logic decision circuits.